1. Field of the Invention
The present invention relates to a continuous casting immersion nozzle for pouring molten steel from a tundish into a mold. More specifically, the present invention relates to an immersion nozzle used for high-speed casting of medium-thickness slabs (about 70 mm to about 150 mm thick).
2. Description of the Related Art
With the trend toward faster continuous casting aimed at increasing productivity of slabs, Japanese Unexamined Patent Application Publication No. 57-106456, for example, discloses as an immersion nozzle that advantageously fits increasing throughputs of casting steel products, an immersion nozzle having a plurality of small holes disposed in the bottom (See FIG. 15). The immersion nozzle may be used with no difficulty in continuous casting when the throughput of cast slabs (pouring rate) is 1 m/min to 1.5 m/min.
Japanese Unexamined Patent Application Publication No. 7-232247 discloses an immersion nozzle for continuous casting including a cylindrical body, the body having a pair of outlets disposed in the sidewall of a lower section thereof and a slit opening formed in a downwardly tapered lower section thereof. The outlets and slit opening are designed to decrease defects in the cast steel products caused by entrapment of inclusions (See FIG. 16A, FIG. 16B). In this immersion nozzle, the bottom is fully opened with the slit opening to make a large open area.
International Publication No. 2005/049249 discloses an immersion nozzle including a tubular body, the body having a pair of opposing lateral outlets in the sidewall of a lower section thereof. The lateral outlets each are divided by one or two inward horizontal projections into two or three vertically arranged portions to make a total of four or six outlets (See FIG. 17A, FIG. 17B). The publication describes that the immersion nozzle permits inhibition of clogging and generation of more stable and controlled exit-streams which are more uniform in velocity and in which spin and swirl are significantly reduced.
In the conventional immersion nozzles that have a pair of outlets disposed in the lower sidewall of the tubular body, larger amounts of the exit-streams issue from the lower portions of the outlets, which results in imbalance in amounts between the exit-streams that issue from the lower portions and the exit-streams that issue from the upper portions of the outlets. With a rise in the throughput, this imbalance increases to form negative pressure in the upper portions of the outlets, thereby possibly allowing the molten steel in the mold to flow into the nozzle through the upper portions of the outlets. This leads to excessive velocities of part of the molten steel streams impinging on the narrow sidewalls of the mold, which in turn causes increased velocities of the reverse flows that impinge on the narrow sidewalls and turn back. The increased velocities of the reverse flows raise the level fluctuation at the surface of the molten steel in the mold, resulting in asymmetric streams on the right- and left-hand sides of the immersion nozzle.
The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide an immersion nozzle for continuous casting, particularly for high-speed continuous casting of medium-thickness slabs, which nozzle permits a reduction in the drift of molten steel flow in the mold and a reduction in the level fluctuation at the surface of the molten steel to improve the quality and productivity of slabs.